Products designed around thermoplastic polyester resins are in great demand because of the tremendous versatility of these resins and their inherent properties.
The term thermoplastic polyester refers to high molecular weight polyester and copolyesters of glycols and dibasic acids such as terephthalic or isophthalic acid. These are described for example in U.S. Pat. Nos. 2,465,319 and 3,047,539. Because of its rapid crystallization rate, polybutylene terephthalate (PBT) has been preferred for injection molded articles while polyethylene terephthalate (PET), with its very slow crystallization rate, has largely been used in packaging. Nucleating agents have been used to increase the crystallization rate, and post molding heat treatment (annealing) can be used to crystallize molded articles made of PET. The crystallized PET tends to higher tensile strength, stiffness, and heat resistance compared to uncrystallized PET. Nucleating agents and/or annealing are generally not required to crystallize molded PBT articles.
In order to improve the overall property spectrum of PET, the resin has often been blended with numerous other resins. Specific additives are also incorporated to increase the general level of the impact strength of the PET containing blends as well as the consistency of the impact property in a molded article.
Illustrative of the specific additives or impact modifiers that have been proposed to be used with thermoplastic polyester resins are the family of modifiers comprising polyalkyl acrylates, methacrylates, and/or ethacrylates (Brinkmann et al; U.S. Pat. No. 3,591,659) typical of which is Acryloid KM-330, a polyacrylate resin sold by Rohm & Haas Co.
Certain rubber resin graft copolymers have also been proposed to effect impact resistance of polyester resins. In U.S. Pat. No. 3,919,353 (Castelnuovo et al) EPDM grafted with common ethylenically unsaturated monomers is disclosed to be an effective impact modifier. Polybutadiene is taught to be the preferred elastomer. All of the examples use polybutadiene grafted with styrene-methyl methacrylates and the elastomer is crosslinked.
One of the resin blends preferred for its overall balance of molded properties is that comprising polyesters and polycarbonate (PC) resins. It should be noted that the impact strength at room temperature of most polycarbonates is relatively high. For example, the Notched Izod Impact at Room Temperature (NIRT)(ft-lb/in.) of numerous commercial polycarbonates such as LEXAN 103 (trademark of General Electric) is often in the range of from about 16 to 20 ft-lb/in. However, to obtain maximum chemical resistance of polyester/polycarbonate blends, it is desirable to keep the polycarbonate content low. Furthermore, polycarbonate resin is susceptible to mechanical stress cracking which can be exacerbated by exposure to unfavorable environments such as the presence of solvents or solvent vapors. This tendency to stress crack provides an additional reason to utilize as little polycarbonate resin as possible in these polyester blends.
When adding impact modifiers to polyethylene terephthalate/polycarbonate blends, susceptibility to the effects of ester interchange on the final blend properties must also be taken into account. Blends under high shear conditions that contain certain impact modifiers, especially low molecular weight esters, tend to break the modifiers down which, in turn, react with the polymers realizing blends that have unpredictable properties. The ester interchange reaction can be minimized, of course, by the use of inhibitors and/or by lowering the shear level.
Various blends based on polyester resin, polycarbonate resin and graft copolymer rubber-resin materials have heretofore been proposed (e.g., U.S. Pat. Nos. 3,591,659; 3,919,353; 4,022,748; 4,034,013; 4,034,016; 4,044,073; 4,096,202; 4,260,690 and 4,280,949.
U.S. Pat. No. 4,257,937 teaches the impact modification of polyester/polycarbonate blends via a polyacrylate resin such as Acryoid KM-330 and 7709-XP (trademarks of Rohm & Haas Co.)
U.S. Pat. No. 4,172,859 (Epstein) discloses blends of polyesters, polycarbonate and an impact modifier which can comprise various monomers, combinations of which can include EPDM.
U.S. Pat. No. 4,493,921 represents another step in the continuing effort to improve the level of performance of polyester-type resin/polycarbonate-type blends via the addition to polybutylene terephthalate (PBT)/PC blends of an impact modifier comprising an ethylene-propylene-non-conjugated diene (EPDM) rubber spine grafted with either a styrene/acrylonitrile monomer mixture, a styrene/methyl methacrylate monomer mixture or a methyl methacrylate monomer mixture.
One of the art recognized difficulties realized in the impact-modification polyester art is the unpredictability of the modifier effect, i.e., a modifier that enhances the properties of one polyester such as polybutylene terephthalate (PBT) or a PBT resin blend, may have little property modification in a different polyester or polyester blend. For example, certain polyacrylate modifiers such as Acryloid KM-330, while capable of significantly improving the impact properties of a PBT/PC resin blend, will have much less effect on a PET/PC blend.